1. Field of the Invention
The present invention relates to a method and an apparatus for accurately measuring, optically, the thicknesses of free-standing or static thin objects. In particular, the present invention relates to a method and apparatus for measuring the thicknesses of free-standing transparent, thin objects such as semiconductor wafers.
2. Background of the Prior Art
The thickness measurements of conventional semiconductor wafers is typically performed by micrometers or by noncontacting capacitance techniques. However, the use of micrometers and noncontacting capacitance techniques has a considerable number of drawbacks.
For example, the use of micrometers for measurement of thicknesses of semiconductor wafers occasions considerable amounts of mechanical damage to the wafer surface--particularly in the case of thin wafers. On the other hand, the disadvantageous aspects of using noncontacting capacitance techniques for measuring thicknesses of semiconductor wafers is that capacitance techniques are not sensitive for wafer thicknesses of 50 microns or less . . . and capacitance techniques also place restrictions on the wafer resistivity.
The disadvantages attendant to measuring the thicknesses of semiconductor wafers come at a time when there is increased interest in very thin semiconductor wafers or films for a variety of devices, including, but not limited to, strain gauges and photovoltaic devices.
A device for contactless measurement of the dimensions of objects is disclosed in U.S. Pat. No. 4,914,307, and comprises: an opaque hollow body having an opening for the introduction of the object to be measured, and at least one radiation emitter diode and a linear photovoltaic cell to generate an electrical signal indicating the quantity of radiation received. The emitter diode and the photovoltaic cell are arranged in the cavity of the body in a common plane perpendicular to the direction of introduction of the object to be measured, so that when the object is introduced into the body in the predetermined direction between the emitter diode and the photovoltaic cell, a shadow is projected that is a function of the transverse dimensions of the object. However, the device of this patent only senses edges or shadows of the objects being measured.
U.S. Pat. No. 4,732,473 discloses apparatus for and methods of determining the characteristics of semiconductor wafers, wherein the method entails providing a light beam, focusing the light beam at an entrance pupil, positioning a semiconductor wafer at a particular focal distance from the entrance pupil for reflection of the light beam by the semiconductor wafer, passing the reflected light beam through a pin hole; focusing the reflected light beam at the pin hole, and positioning a light sensor at a particular distance from the pin hole to obtain production of signals by the light sensor consistent with the characteristics of light sensed by the sensor. The apparatus of this patent uses reflected light in order to sense objects on or the topography of a surface, and does not directly address measuring thicknesses as such.
An apparatus for and method of measuring material thickness is disclosed in U.S. Pat. No. 3,732,016. The apparatus includes: a light source projecting light toward the material, a light detector receiving the projected light from the material and providing electrical signals corresponding thereto, a converter receiving the electrical signals and providing oscillating output signals having a frequency dependent upon the electrical signals, and a counter receiving the output signals to represent the thickness of the material by the count stored therein. However, the apparatus uses variations in transmitted light, and the film is continuously moving pass the light source and photocell. In all cases, the signals from the photocell are converted to oscillating signals over a specified period of time, and counting of the oscillations over the time period allows determination of the thickness. The apparatus does not appear to be related to measurement of thicknesses of a static object.
U.S. Pat. No. 5,130,556 discloses an optoelectronic position-sensitive detector used to measure fiber thicknesses. The position-sensitive detector is disposed at one side of a sensing region, wherein the light source is directed toward the position-sensitive detector to illuminate the same, so that an object present in the sensing region blocking a portion of the light effects a shadow on at least a portion of the position-sensitive detector, and electronic means receive and compare output signals from the position-sensitive detector to obtain a resultant single output signal, which indicates the lateral position or size variation of the object within the sensing region. However, the detector in this patent only senses edges of an object in order to deduce its width or thickness.
A method of and apparatus for measuring film thicknesses in the area of 10 nm is disclosed in U.S. Pat. No. 5,120,966, and comprises means for applying light in the ultraviolet region toward a transparent thin film on a sample, means for measuring energy of light reflected by the object sample; and means for obtaining the thickness of the transparent thin film on the basis of the energy value measured by a measuring means. This method uses reflected light to determine thicknesses of a transparent film on a substrate. It is not used for determining thicknesses of free-standing or substrate-free objects.
Therefore, there is a need extant in the semiconductor art to evolve and devise apparatus for and methods of bypassing the problems or drawbacks that accompany conventional methods for determining thicknesses of semiconductor wafers. Toward these ends, the present invention utilizes light absorption as a gauge of thickness for measuring the thicknesses of semiconductor wafers.